LONDON — One of the militants in the Paris attacks traveled to Syria from his hometown in France and back, officials said, even after his passport had been confiscated and he had been placed under judicial oversight. So did another, despite having been arrested eight times in petty crimes and having been listed as a national security risk in France.

Even the man suspected of organizing the massacre on Friday, Abdelhamid Abaaoud, a well-known figure in the Belgian jihadist scene, is believed to have traveled between Islamic State-controlled territory and Europe a number of times — including for an attack plot in Belgium in January.

The Paris attacks, the deadliest in France to date, have sharpened the focus on the inability of security services to monitor the large and growing number of young European Muslims who have fought alongside the Islamic State or to spot terrorist plots in their early stages, even when the participants are well known to them.

It appears so far that as many as six of the assailants who killed 129 people with guns, grenades and suicide bombs at six sites last Friday were Europeans who had traveled to Syria and returned to carry out attacks at home — precisely the nightmare scenario security officials have been warning about for the past two years.

“This is the attack everyone was worried about, and it finally happened,” said Louis Caprioli, who was the deputy head of France’s domestic antiterrorism unit from 1998 to 2004. “A high-casualty attack on multiple soft targets executed with apparent military know-how.”

The failure to detect the plot despite warnings has raised old questions with new urgency: Can the threat be contained in Europe without a sharply increased military effort to destroy the Islamic State on its home turf in Syria and Iraq? Is Europe’s informal system of intelligence sharing adequate in the face of such threats? And do intelligence services need even more resources and surveillance powers?

The latest attacks appear to validate concerns that both in scale and scope, the conflict in Syria represents a novel security threat to Western countries and Europe in particular: The number of Europeans drawn to fight jihad there has swelled to more than 3,000 in a little over two years. And the vast territory controlled by the Islamic State offers militants the opportunity to train in combat and bomb-making, which they can apply to terrorist operations at home.

More than 1,000 French citizens and 600 Germans are believed to have traveled to Syria to join the Islamic State, also known as ISIS or ISIL. Belgium, with its population of about 11 million, has 520 fighters in Syria, the greatest number of fighters per capita. More British Muslims have joined the Islamic State — about 750 — than are currently enrolled in the British armed forces, according to Shiraz Maher, a senior research fellow at the International Center for the Study of Radicalization at King’s College London, who has been monitoring the social media accounts of Western jihadists over the past two years.

The Islamic State, he said, “has mobilized the largest volunteer army of Sunni fighters in recent history.”

The threat from returning jihadists is not new. In the 1980s, Europeans fought in Afghanistan against the Soviets, and in the ’90s some went to Bosnia. Since then, Iraq, Somalia, Yemen and Afghanistan — again — have been destinations. One of the brothers who shot 12 people at the satirical newspaper Charlie Hebdo is believed to have spent time in Yemen. The leader of the London bombings in July 2005 had been to Afghanistan three times.

But the traffic to and from Syria is much heavier, because of its easier access to Europe and a powerful Islamic State propaganda machine that paints a promised land of religious virtue, Muslim community and righteous revolution with sound-bite religion, analysts say.

Not all volunteer fighters returning from conflict become terrorists. Some academic research suggests that one in 10 do, while other sources say the ratio is as high as one in four. Either way, the end result is a terrorist threat expanding at a rate that alarms security experts.

“The threat we are facing today is on a scale and at a tempo that I have not seen before in my career,” said Andrew Parker, the director general of Britain’s domestic security service, MI5, in a lecture last month. Over the past year, he said, his service has foiled six attacks in Britain alone.

Intelligence officials frequently complain that their ability to eavesdrop on suspects is increasingly being abridged by concerns about personal freedoms. The problems have only increased, they say, with the availability of sophisticated encryption technology in instant messaging services like WhatsApp and iMessage, and in less mainstream platforms like Telegram.

Others bemoan a lack of trust and intelligence sharing in Europe. One senior Belgian counterterrorism official said that Turkey routinely failed to respond to requests for information, and suggested that this might have played a role in suspects’ slipping through the cracks. A Turkish official, however, said that his agency had twice told France the name of one of the Paris attackers, most recently in June, but did not hear back until after the massacre.

“What we need is the systematic sharing of information in real time,” said Mr. Caprioli, the former counterterrorism official.

Pointing to an initiative after the Sept. 11 attacks to form a Europe-wide intelligence task force with representatives from the biggest services that went nowhere, Mr. Caprioli said that some form of institutionalized sharing of electronic and human intelligence was “essential,” particularly given Europe’s open borders. The Schengen area of passport-free travel, he said, made it easier for the Paris attackers to prepare in Belgium.

The biggest challenge, counterterrorism experts and officials said, was not so much identifying those who represent a potential threat, but knowing whom to put under the tightest surveillance. In France alone, about 3,000 people are considered potential threats, officials said.

“It’s an issue of volume,” said Raffaello Pantucci, the director of international security studies at the Royal United Services Institute. “The system is overwhelmed. There are so many individuals and cases they are worried about by now, historic and current, that they cannot keep up.”

As in other recent attacks, many of the Paris militants were well known to the authorities, but they still managed to slip through the net.

Samy Amimour, a 28-year-old French bus driver whom officials identified as one of the gunmen at the Bataclan concert hall, was charged with terrorist conspiracy in 2012. Suspecting he was planning to go to Yemen to fight, the authorities confiscated his passport and placed him under judicial control, meaning he was barred from traveling and had to report regularly to the authorities.

Nevertheless, a year later Mr. Amimour managed to make his way to Syria undetected. At least once a month he communicated with his family via Skype, the French newspaper Le Monde reported last December in an article about how his 67-year-old father had traveled to Syria to try to bring him back. But the father said that when he returned, the police did not try to debrief him.

Another attacker at the concert hall, identified as Ismaël Omar Mostefaï, 29, had a “fiche S,” a police file denoting him as a potential security threat. But he, too, was able to travel to Turkey in 2013, and is believed to have crossed into Syria from there.

Two brothers the police say were involved in the attacks — Salah Abdeslam, who is still at large, and Ibrahim Abdeslam, who detonated a suicide bomb at a cafe on Friday — are also believed to have spent time in Syria, although investigators are still trying to determine when. When Ibrahim tried to go in February, he reached only Turkey before he was sent back, a spokesman for Belgium’s federal prosecutor’s office said on Wednesday.

“There’s always a possibility that he went later,” the official said. Syria is so accessible, he added, that “we can’t very realistically keep an eye on everyone” who travels there.

Perhaps the most striking example of the apparent ease with which jihadists can move between Europe and Syria is the man thought to have orchestrated the attacks, Mr. Abaaoud, who was killed in an early-morning raid Wednesday. He is believed to have first gone to Syria in 2014, and may have returned there twice undetected. The last time he claimed to have returned — after escaping a raid by the Belgian police in January — he gloated to an Islamic State magazine about making it back “despite being chased by so many intelligence agencies.”

One result of the traffic between Europe and Syria is that many of those returning have had training, suggesting that the litany of failed attempts with crude explosive devices in Europe over the last decade may be a thing of the past.

In August, French security services interrogated a young Frenchman returning from Syria, Le Monde reported. The man described the Islamic State as a “factory” of terrorist plots.

Asked what attack might be planned, he said: “All I can tell you is that it will happen very soon. They really want to hit France and Europe.”

Dan Bilefsky contributed reporting from Paris, and Kimiko de Freytas-Tamura from Brussels.

Get news and analysis from Europe and around the world delivered to your inbox every day with the Today’s Headlines: European Morning newsletter. Sign up here.

A version of this article appears in print on , on Page A14 of the New York edition with the headline: Assaults Highlight Jihadists’ Easy Path to Syria and Back. Order Reprints | Today’s Paper | Subscribe